Mumbai University > Information Technology > Sem 3 > Database Management System

Marks: 5M

Year: May 2015, Dec 2015

1. Introduction:

• The participation constraint specifies if an entity’s existence depends on its being related to any other entity via the relationship type.
• This constraint specifies the minimum number of relationship instances that each entity can participate in, and is sometimes called the minimum cardinality constraint.

• Total Participation:

• The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R.

• Consider two entities EMPLOYEE and DEPARTMENT related with the help of relationship WORKS_FOR.
• If a company policy states that every employee must work for a department, then an employee entity can exist only if it participates in at least one WORKS_FOR relationship instance. Thus, the participation of EMPLOYEE in WORKS_FOR is called total participation, meaning that every entity in the total set of employee entities must be related to a department entity via WORKS_FOR.
• Total participation is also called existence dependency.
• Total participation is displayed as a double line in ER diagram connecting the participating entity type to the relationship

1. Partial Participation:

• If only some entities in entity set E participates in relationships in R, the participation of entity set E in relationship set R is said to be partial.
• Consider two entities EMPLOYEE and DEPARTMENT related with the help of relationship MANAGES.
• Here, not all employees are expected to manage a department, so the participation of EMPLOYEE in the MANAGES relationship type is partial.
• This means that some or part of the set of employee entities are related to some department entity via MANAGES, but not necessarily all.